TOPIC:

I have been working on my 1981 KZ750h for about a month now. I will be upfront and admit I am not a great mechanic. I have most of the issues with the bike resolved however I have one issue with the spark that I can't figure out. Sometimes the bike fires up and runs on all four cylinders with no issue but from time to time it only starts and runs on cylinder 2 and 3. If it starts up on all four cylinders it keeps running on four with no fail. If it starts on two sometimes revving the engine causes the other two to fire and sometimes it doesn't. I have another thread that addresses this to a point but I thought that i would start a thread that addresses only this. If anybody has seen something similar i would appreciate the input. The ignition coils have been tested at a shop and are good. The plugs are all new and gapped correctly. The IC igniter seems to work because the bike runs properly some of the time. The left pick-up coil seems to not work at times but this could be caused by many different things. The right pick-up coil always works. All of the connections have been cleaned and sanded or filed. Could this be a loose wire or a bad pick-up. It is troubling because it works some of the time.

if you are having issues with the left pick up coil, seems to me that is the place to start,, I dont know which cyl it controls but they both need to be working all the time.
also check the voltage to the coils at the coils,, make sure it is not weak

The left pickup coil is for firing the 1 & 4 plugs. Since these are the plugs that sometimes don't fire, I suggest you replace the stock stuff with a Dyna S. For under $150, your problems will be resolved.

I dunno if this helps or not but I had a similar experience last wknd. Here's the thread:
kzrider.com/index.php?option=com_kunena&...d=4&id=205004#205004

I had a similar problem on my 750. Both coils would fire from idle to redline when the bike was cold. When the bike would get to temperature, one of the coils wouldn't fire below 3000rpm. The coil would fail when hot. I could pull both boots of the sparkplugs when hot and no change in running. Made it hard to ride around town on 2 cylinders, then jumping on to 4. New coils did the trick. I hope you find the problem.

del

Thanks for the advice. I had cleaned all the electrical connections about 2 weeks ago thinking it would help. The problem is getting more sporadic and random now. It has changed now so that the two cylinders just cut in and out whenever, even when I am riding, which is dangerous.

Well, if you are not up to buying a Dyna S, have you considered swapping coils... just unhook them and swap them and reconnect everything. If the problem MOVES to the 1 & 4 plugs not firing, you have determined you have a bad coil. If the problem stays on the 2 & 3 plugs, you have determined either the pickup coil OR IC Igniter is bad.

While you have the coils out, look at them carefully for cracking. Test both primary and secondary resistance. If the problem is intermittent, testing at a shop is more or less a waste of time. Inside the coils are two sets of copper windings. These windings are coated with plastic to insulate. If the insulation melts a tad, you will get a short. The short could well be intermittent.

Back to swapping the coils? Don't rely on a shop to tell you if the coils are good... test their resistance. If the resistance is out of spec, this can indicate a short, which as I said, may be intermittent.

I have swapped coils and the problem has stayed with the 1 and 4 cylinders. Is the Dyna S ignition worth the extra money over buying a set of used pick-ups? I have determined that the problem is definitively with the pick-up. When I test them with the ohmmeter I get no signal through the one pick-up most of the time. I am not saying that there isn't another problem down the line just that it looks like the pick-ups need to be replaced. Does anyone know why the pick-up will work some of the time?